Photosensitive electrode and method for producing same



Jan- 29, 1957 R. G. sTouDENHEIMER 2,779,383

PHOTOSENSITIVE ELECTRODE AND METHOD FOR PRODUCING SAME Filed Dec. 16, 1952 00 I BY wm/fzf/varH-A'A/dsfeaMs ATTO RNEY United States Patent@ PHOTSENSITIVE ELECTRODE AND METHOD FOR PRODUCING SAME Richard G. Stoudenheimer, Lancaster, Pa., assgnor to Radio Corporation of America, a corporation of Delaware The invention is directed to a photosurface, and more specifically to a photoemissive cathode electrode `to be used in photo tubes.

Semitransparent antimony films on a transparent supporting surface and used as photocathodes are formed from a film ofantimony upon which has been deposited a thin film of' ccsium. Such photosensitive films have spectral responses varying over a range between 3,000 A. units to 6,400 A. units. The responses are normally peaked in the blue at about 4,000 A. These photocathodes are used in photomultiplier tubes intended for use as scintillation counters to detect and measure nuclear particle radiation and in other applications involving lower level, large-area light sources. The cesiated-antimony photocathode, because of its high sensitivity to blue-rich light and its negligible sensitivity to red radiation, is also suited for use with organic phosphors, such as anthracene, as well as inorganic materials such as titanium-activated sodium iodide. A further use of such a photosurface is in color television pickup cameras in which the camera tube utilizing a cesiated antimony photosurface has utility in pickup of the blue component of the scene televised. The usefulness of such photosensitive devices depends upon their light sensitivity. That is, it is desirable that as strong a signal as possible be produced from a minimum light source. Since photosurfaces of the type described are normally formed by the evaporation of volatile materials, such as antimony onto the cathode support, it is ditiicult to bake such tubes at the high temperatures required to provide high vacuum, since the volatile material, used subsequently for forming the photosurface, is vaporized at the high temperatures. Accordingly, then the baking temperature is kept somewhat lower than optimum with a resulting loss in performance of some characteristics in the finished tube due to the presence of deleterious gases which are given ofi from tube surfaces during tubeoperation.

lt is, therefore, an object of this invention to provide a photosensitive device having improved performance.

it is another object of the invention to provide a photosensitive tube having a photocathode formed of antimony and cesium and of improved sensitivity.

lt is another object of my invention t provide a phototube having a photocathode formed from antimony and cesium in which the tube has improved performance due to a high vacuum within the tube.

Specifically, the invention relates tov a photoemitting `tube having an improved semi-transparent photocathode formed from an alloy of manganese and antimony, which is chemically combined with oxygen and cesium. It

has beenfound that a phototube made in the manner to be described has an improved performance. The use o f a manganese antimony alloy provides improved tube characteristics and also enables the use of a high vacuum during tube operation with improved tube operation.` f Figure l is a sectional view of a phototube `having a photocathode, in accordance with the invention.

Figure 2 is a sectional view of the opposite side of the tube of Figure 1.

Figure 3 is a cross-sectional view of the tube of Figure 1 and along the section lines 3-3 of Figure 1.

Figure 4 is a spectral sensitivity curve of a photocathode utilizing the photoemissive surface of the invention.

Figure 5 is a photosensitivity curve of a plurality of photosurfaces formed with dierent percentages of the alloy used, in accordance with` the invention.

Figures l and '2 disclose a phototube of a type which uses a photocathode formed in accordance with the invention. On the inner surface of the envelope 10 of the tube there is formed, to be described below, a semi-transparent photoemissive film 12 to serve as the source of photoelectrons during tube operation. The photocathode film 12, in the particular tube shown, extends around the cylindrical wall portion of the tube from diametrically opposite points of the tube envelope and from substantially one end of the envelope to the other end, as shown in Figures 2 and 3. The photocathode surface is connected to a source of potential through strips 14 of evaporated metal at the longitudinal edges of the film 12. One of the metal strips is connected to an electrode lead 16 spring-pressed into contact with the conductive strip le, as shown in Figure 2. Lead 16 is connected in turn to a pin 17 sealed through the base of envelope 10.

Positioned along the axis of the tube is a focusing electrode 1S consisting of a large tiat plate extending along its longitudinal edges close to the side walls of the tube envelope itl. One surface of the plate 18 faces the semicylindrical photosurface 12.` Spring fingers 20 resiliently space plate 1S from the side walls and also serve to malte Contact with the conducting strips 14 whereby the focusing plate 18 and the photocathode film 12 are maintained at a common potential during tube operation. Substantially at the center of plate 18 there is formed a rectangularly shaped aperture 22. Positioned across the aperture 22 and slightly spaced therefrom in the direction of the photocathode film 12 there is a grid wire 24 mounted at each end on a lead wire 2u `held in an insulating sleeve 28 passing through the focusing plate 18. At either end of the focusing plate is fixed a semicircular plate 30 extending toward the photocathode film 12 to a point closely spaced from the tube envelope wall.

During tube operation, the grid wire 24 is kept at a high positive potential in the order of 750 volts positive relative to the photocathode lm 12, which may be held at ground potential. Photoelectrons emitted by film 12 will normally be urged toward the positively charged grid Wire 24. The focusing plate 1S together with the two end walls Sil forms a grounded cage, substantially enclosing the photocathode filmlZ so that there will be little or no tendency for photoelectrons emitted from surface 12 to land on the parts of' the cage. rThus, photoemission from any portion of the semi-cylindrical photocathode surface 12 will be directed toward the positive grid wire 12.

. Positioned on the other side of aperture 22 from the grid wire 12 is a multiplier unit 32. The multiplier unit is of the type disclosed in U. S. iatentV 2,285,126 to Rajchman et al. The multiplier consists substantially of a plurality oi dynotles or secondary electron emitting electrodes which are positioned opposite to each other to collect the emission from a preceding secondary emitter. The multiplier 32 is positioned with the: first dynode 34 opposite `the aperture 22 through focusing plate 18. Photoelectrons from film 12 directed toward the positive grid wire 2e will pass through aperture 2?. and strike dynode 34 to provide a secondary emission therefrom,

emission from which passes to` successive dynodes 38 etc.

Thus, photoelectrons passing into the multiplier section are amplified by this emission from each dynode so that the electron ow from the final dynode 40 to the collector 42 is a large amplification of the original photoemission in the order of 600,000. The multiplier section shown in Figure 3, for example, is illustrative and is not meant to be limiting, since other multiplier units are known and may be also utilized, in the tube described.

Phototubes, of the type described and shown in Figures 1 3, can be utilized in scintillation counters, for example, for detecting and measuring nuclear particle radiations. Such a tube, in one application, is slipped into a portable probe, which includes, a cylindrically curved phosphor surface formed of such material as anthracene or thalliumactivated ,sodium iodide, which materials provide a high blue-rich light when excited on bombardment by nuclear particles or radiation. Any `particle or radiation striking the phosphor produces a scintillation of light which will excite the photocathode film 12.

The photosurface 12 is normally formed of a film of antimony, which may be put down onto the envelope surface by evaporation. Toy sensitize the antimony film, cesium metal is vaporized within the tube envelope and condenses on the antimony surface. Care is taken to permit the condensation of cesium only to an extent which will produce maximum sensitivity. Such a cesiated antimony photosurface is one which has a spectral response covering a range of about from 3,000 A. units to 6,200 A. and has a maximum response at approximately 4,000 A., which is in the blue region of the spectrum.

A cesiated antimony photosurface of the type described above and having improved sensitivity is that disclosed in the copending U. S. application Serial-#219997, liied on April 9, 1951 by J. I. Polkosky, now U. S. Patent 2,676,282, issued April 20, 1954. The improved photosurface disclosed in this copending application has a thin film of manganese oxide rst formed on the glass tube surface prior to the deposition of the antimony lm. The use of manganese oxide with a cesiated-antimony photosurface provides an increased sensitivity and a spectral response which is shifted toward the red, depending upon the degree of oxidation of the manganese lilm. This improved cesiated antimony photosurface has a sensitivity which averages around 40 microamperes per lumen, with recorded maximum sensitivity in the order of 100 microamperes per lumen. Such a sensitivity compares favorably with the sensitivities of other photosurfaces known at the present time.

The copending application of Polkosky describes in detail the formation of the improved cesiated antimony photosurface. Briefly the method is that in which the tube envelope is exhausted and the tube baked at an elevated temperature between 260JV C. and 280 C. for one half hour. After the tube is allowed to cool to ambient temperature, a pellet of manganese metal is evaporated from a heated filament onto the transparent photocathode support to form a film of metal. This manganese lm is oxidized and then the antimonyis subsequently deposited over the iilm from an evaporation of antimony metal pellets from a heated filament. It has been found in the tubes of this type, however, that they can not be baked at the optimum temperature to produce the degree ot' vacuum necessary for some applications. If the tubes are baked at much higher than the 280 set forth above, the antimony pellets are vaporized during the baking of the tube, so that the subsequent formation of the photosurface is prevented. Since it is necessary to bake the tubes at lower temperatures, there can not be established the degree of vacuum required for optimum tube sensitivity, within the tube as occluded gases from the tube envelope and electrode-structures will be driven out into the discharge space of the tube during operation.

i If gas is present in a multiplier tube, then, an electron vcurrent through the multiplier will ionize the gas. For

example, if a strong ash of light starts a surge of electrons through the tube, a large number o positive ions are formed which drift towards the cathode. The time for the positive ions to reach the cathode or preceding dynode is much greater than the time required for the original surge of electrons to pass in turn through all the dynode stages. The electron transit time in a type of multiplier tube described in the above cited application of Polkosky, is about 10-8 seconds. Y et a hydrogen ion requires about l0-6 seconds or 100 times longer to trav-el from the region near Vthe accelerating electrode wire 2d to the cathode. When the hydrogen icn does reach the cathode, it will Create new free electrons at the cathode by secondary emission which will start a second surge of electrons through the multiplier. This second surge of electrons is called an afterpulse or satellite pulse. lt is a false indication of a liash of light at the cathode and seriously interferes with certain types of measurements as for example in scintillation counters. A properly degassed multiplier tube does not exhibit afterpulses. ln television pickup tubes and other tubes forming an image, gas present in the tube forms an ion spot in the center of the picture. This spot is bright initially because positive ions striking the center of the cathode produce secondary electrons. Eventually the central spot turns dark because ion bombardment destroys the cathode surface.

In accordance with the invention, a olue sensitive cesiated antimony photosurface is formed and permits a higher baking temperature for the phototube and a subsequent improved `vacuum for tube operation. The photoc'athode surface 12 is formed in accordance with the invention from a manganese-antimony alloy of 20% to 36% of manganese, with improved results and additional advantages.

The tube of Figures l, 2, and 3 is normally assembled by mounting the multiplier dynodes in their assembled arrangement on ceramic end pieces 21, which insulatingly space the `dynode electrodes from each other and lock them together as a unitary assembly. The multiplier unit 32 then is mounted to the appropriate side of the focusing plate 18 by short rods 43 welded to the metallic structure of the multiplier unit and sealed to glass beads dd which 'in turnare sealed to other glass rods d5 welded or fastened to the plate 1S.. The several dynode electrodes of the multiplier 32 are lixed to lead wires i6 extending toward the lower end of the tube envelope and are in turn respectively xed by welding to base pins 4S sealed through a flat glass 'disc or press 50 sealed 'across a metal ring 52. 46 support the focusing plate 18 together with the multiplier unit 32- from the glass press 50. Mounted on the focusing plate 18 and on the same side as the grid wire 24 are a plurality of filament wires 51, each having one end thereofwelded` to plate V1S and the other end thereof welded to -a lead means 53 extending through a tubular insulator to the opposite side of plate 18 and to a common strip 54, which is connected toone or more of the base pins 48. As shown in Figure l, one of the liiament leads 53 is connected to a separate lead 45. On each lainent wire 51 is a lead 5S oian Ialloy of manganese and antimony, in accordance with the invention.

The envelope portion 10 has a tubulation 52 at its upper end through which ther tube can be exhausted.' At the opposite end the envelope is sealed to a metal ring 57. Theenvelope portion 10 is first formed with the conductive metal strips 14 extending longitudinally Ialong opposite sides of the tube envelope as shown and by evaporating aluminum onto the inner surface of the glass envelope with the remainder of the inner envelope surface vmasked from lthe source of aluminum. The glass press portion 50 together with the plate electrode 18 mounted thereon, in the manner described, is inserted into the open en'd ofthe envelope 10 and the rings 52 and 57 are welded together by a helio-arc weld, for example. The exhaust tabulation S2 is connected to a vacuum system and the Leads I `the dynodes offmultiplier 32 can `be made of silver magnesium or beryllium copperand the tube baked at 400 C.

`te 450 C. If. the presenceof a small amount of residual gas is not harmful for the particular tube application, the gain can be increased considerably by using nickel base dynodes coated with a thin layer of antimony. However, since antimony isvery volatile the temperature of the first bake must be limited to a maximum of about 300 C. A temperature of 270 to 280is commonly used with antimony coated dynodes. `After baking, the tube is removed from the oven and allowed to cool to ambient or room temperature. t .i j

The photosurface 12 of the invention is then made according tothe `following procedure andalso in accordance with the invention. A current ispassed through the filament wires 5l by connecting'respectively base leads 48 of terminal plate 54 and the lead 43 to a source of current. The flow of current through filaments 51 heats the filaments to a temperature sufficiently high to evaporate the manganese-antimony `alloy beads 55. The evaporated alloy` metal passes over onto the cylindrical Wall portion of the envelope and forms a film-like deposit thereon. The endplatestructures 30, aswell as the focusing plate 18 prevent the evaporated material from passing beyond the limits of these plate structures to the other portions of the tube. The evaporation is continued until the light transparency through the envelope wall has been changed to 90% of the .transparencyV of the wall before evaporation. The transparency can be measured by passing a beamA of lightthrough the envelope wall onto a photosensitive light measuring device. Such a method is more fully disclosed inthe copending application of I. J. Polkosky cited above. When the transparency of the film 12 has been dropped to 90%, the evaporation of the alloy pellets is discontinued by cutting off the filament current. Next oxygen is admitted into the envelope through `the exhaust tubulation 52 and to a pressure of between 0.7 to 0.8 mm. of mercury. The thin film of manganese-antimony alloy is oxidized by means of a glow discharge adjacent the film surface. This discharge is established by Wrapping a metal foil around the tube and then connecting themetal foil to a source of radio frequency potential. This sets up a gaseous discharge within the tube envelope which heats the surface of the film in the presence of the oxygen gas and thus oxidizes film l2. The oxidation is -continuedfor about two` seconds or until ,the light transmission to the tilmlZincreases to 95%. `The oxidation step is discontinued and the oxygen pumped out of the tube envelope. Currentis-now again passed through the evaporator lilamentsSl and the manganese-antimony alloy is 4again evaporated onto the surface oftilm 12 until the `light transmission through the surfaceis reduced to 75%. The tube is now baked in an ovenat a temperature of 170 C. to 180 C. lf the construction of the tube permits,

better sensitivity can be obtained if the tube is baked substantiallyat `250 C." During this baking, the cathode iilm l2 is kept' at a temperature between 90 and 150. The baking is continued for l0 minutes until all internal parts of the tube are brought to a constant temperature. At this point cesium metal is introduced into the tube from capsules 5S containing a mixture of cesium chromate and silicon. The cesium metal is released by heating capsules 58 with an external R. F. coil until the cesium metal formed by the reaction of the cesium chromate and silicon is vaporized and passes into the tube envelope. Due to the diiierential heating of the tube parts, the cesium will pass over to the cooler surface of ilm 12 and condense thereon to sensitize iilm 12. The other tube parts held at the elevated temperature will not permit condensation of the cesium metal on their surfaces. Thus, the cesium is prevented for example from depositing on the multiplier 16 :structures to start leakage pathsl over'theinsulationseparating the dynodes.H :r t f;

During the vaporization of the cesium metal, the tube potentials are established on the respective electrodes and lilm 12 is exposed to light, so that any photoelectric current liow from lilm 12 through the multiplier unit 32 can be detected. The cesium vaporization is continued until the photoelectric current produced is at a maximum at which point the cesium vaporization is discontinued. The baking of the tube is continued at 180 for 10-12 minutes, while the tube is kept on the pumps and excess cesium is removed through the exhaust tubulation 52. The tube is then tipped off by sealing the tubulation S2 to provide an evacuated envelope. t

As shown in Figure 4, curve A indicates that the manganese-antimony photosurface provides a spectral response which coversta range from about 3,000 A. units to 7,000 A. units withamaximum response at 4,000 A. units. The photosurface has high sensitivity to blue rich light and relatively negligible sensitivity to red radiation.

It has also been found that with photosurfaces of ythis type there :can be introduced a slight shift in the maximum of sensitive response from the blue toward the red. For example, curve B of Figure 4 illustrates a spectral response of a photolm formed of manganese-antimonf. alloy in the manner described, which is somewhat thicker than that formed as described above and indicated by curve A. However, the optimum thickness of photosurface is that described above and one providing a blue response.

A series of tests have determined that there isl an `optimum ratio of manganese to antimony in the alloy used for making the photosurfaces of the invention. Figure 5 is a curve showing results of multiple tests in which the ratio of manganese to antimony was varied from zero to It can be determined from Figure 5 that as the ratio increases from zero percent of lmanganese in the alloy, the sensitivity increases from a `fair value to a maximum. Alloys containing 45% manganese or more, however, yielded consistently very low sensitivities. Pure t antimony films were also extensively tested which showed that such lilms very rarely reached the high values of sensitivity obtained with the alloy lms. Alloy lrns having a percentage of manganese within the range of 10% to 40% gave from fair to optimum sensitivities. However, the optimum range of the percentage of manganese in the alloy was substantially from 20% to 36%.

Tubes of the type described are normally made, with the use ot' a manganese-antimony alloyl formed from pellet consisting of 70% of antimony and 30% of manganese. The antimony material used is a high grade antimony metal having a percentage composition of 99.8% of a pure antimony, 0.03% maximum of iron, 0.03% maximum of arsenic, 0.2% maximum of lead and 0.01% maximum of copper. Such a sample of antimony should be limited to 0.04% of sulfur, 0.001% of zinc or 0.001% of bismuth. The manganese metal used in the alloy is commercial electrolytic manganese within the chemical limits of 99.9% minimum of manganese and 0.1% maximum of sulfur and iron.

A typical procedure for-forming the antimony-manganese alloy, used in the invention, is, for example, that in which quantities of manganese and antimony are weighed out to make a l5 gram melt of 70% antimony and 30% manganese. The 'materials are placed in a glass ampule, which is sealed to a vacuum system and exhausted to about 0.05 micron of mercury pressure after which the tubulation of the ampule is tipped oit and sealed. One end of the ampule is heated until the metals become molten at around 900 C. The ampule is shaken to insure a good mixing of the metals. The heating is repeated a second time and the ampule is quenched in ice Water as rapid cooling of the melt is a requisite of the formation of a uniform alloy composition. The alloy slug is'. etched in. hydrochloric acid to'remove oxides and then Washed several times. The alloy slug than can yhebrokeninto small pieces and melted around the lament wires T prior to mounting them within the tube.

There are a number of advantages in the use of manganese-antimony alloy. Tubes utilizing a photosurface formed from such an alloy can be baked at higher ternperatures to remove more of the occluded gases in the surface of the tube envelope and tube electrodes as described. This results in a higher vacuum within the tube envelope during its operative life and improved characteristics for some applications. An additional advantage, in the use ot' manganese-antimony alloy, is the ease with which the alloy can be made and attached to the lilaments 51 prior to the formation of the photosurface. in

tubes of the type which utilize separate manganese and antimony iilainents, it is necessary to use more filaments and beadsV than is required when the atloy is used. Also7 manganese pellets require a high temperature to melt and attach to the tungsten evaporator. The high temperature makes the tungsten wire very brittle and gives rise to frequent breakage of the evaporators.

The novel manganese-antimony alloy photosurface of the type described above has particular use in photodevices requiring high blue sensitivities, such as multipliers intended for use in scintillation counters to detect and measure nuclear particle radiation as well as those which are used with organic phosphors producing a bluerich light. Such photo-multiplier tubes also have application in systems utilizing a ying spot scanning, in which the iiying spot tube produces a blue, short-persistent light. The new photosurface has also found application in television camera pickup tubes of the image orthicon type for use in color television systems where high sensitivity to the blue component of the picture is desired.

The invention is not limited to the use of manganese alloyed with antimony, since other metals, such as nickel, iron, cobait and chromium, can also be used to form an alloy with antimony in the manner described. For example, a photoemissive surface made with a nickelantimony aloy has provided a spectral response similar to that described above for the manganese-antimony photosurface. The nickel-antimony alloy Was formed by evaporation from a tungsten iilament, which was first plated with manganese and then with nickel in the desired proportions. The sensitivity ot the photosurface was in the order of 3041-0 microarnperes per lumen.

While certain specic embodiments have been illustrated and described, it will be understood that various changes and modications may be made therein Without departing from the spirit and scope of the invention.

What is claimed is:

l. The method of making a photosensitive electrode, said method comprising the steps of, evaporating upon a support having a normal transparency a layer of 'manganese-antimony alloy until the light transmission through said support is reduced to substantially 90% of normal transparency, oxidizing said manganese antimony alloy layer until the light transmission through said support is 95% of normal transparenc evaporating a second layer of said alloy upon said oxidized rst alloy layer until the light transmission through said support is substantially of normal transparency, and sensitizing said oxidized manganese-antimony alloy surface by condensing cesium vapor upon said second alloy layer.

2. A photosensitive electrode comprising, a transparent support, a semi-transparent tilm on a surface of said support, said lm including an alloy of antimony and a metal selected from the group consisting of chromium, manganese, iron, cobalt and nickel, said alloy being combined with oxygen and cesium, the percentage of the weight of antimony in said alloy being from 64% to 80%.

3. A photosensitive electrode comprising a transparent support, a semi-transparent lm on a surface of said support, said film including a mixture of antimony and a metal selected from the group consisting of chromium, manganese, iron, cobalt and nickel, the Weight percent of said metal being in the range of 10 to 40, said film also including oxygen and cesium;

4. A photosensitive electrode comprising a transparent support, a semi-transparent lm on a surface of said support, said lm including a mixture of manganese and antimony, the Weight percent of manganese being between 10 and 40, said lm also including oxygen and cesium.

5. A photosensitive electrode comprising a transparent support, a semi-transparent lm on a surface of said support, said lrn including a mixture of manganese and antimony, the Weight percent of manganese being between 20 and 36, said lm also including oxygen and cesium.

6. The method of making a photosensitive electrode, said method comprising the steps of, evaporating upon a support having a normal transparency a rst layer including manganese and antimony until the light transmission `through said support is reduced to substantially percent of normal transparency, exposing said first layer to oxygen until the light transmission through said support is percent of normal transparency, evaporating a second layer of manganese and antimony upon said first layer until the light transmission through said support is substantially 75 percent of normal transparency, and condensing cesium vapor upon said second layer.

7. The method of making a photosensitive electrode comprising the steps of evaporating a manganese-antimony alloy containing 20 to 36 Weight percent of manganese upon a support having a normal transparency until thc light transmission through said support is reduced to sub- References Cited in the file of this patent UNITED STATES PATENTS 1,906,448 De Boer et al May 2, 1933 2,122,860 Gorlich July 5, 1938 2,218,340 Maurer Oct. 15, 1940 2,254,073 Klatzovv Aug. 26, 1941 2,527,981 Bramley Oct. 3l, 1950 2,676,282

Polkosky Apr. 28, 1954 

3. A PHOTOSENSITIVE ELECTRODE COMPRISING A TRANSPARENT SUPPORT, A SEMI-TRANSPARENT FILM ON A SURFACE OF SAID SUPPORT, SAID FILM INCLUDING A MIXTURE OF ANTIMONY AND A METAL SELECTED FROM THE GROUP CONSISTING OF CHRONIUM, MANGANESE, IRON, COBALT AND NICKEL, THE WEIGHT PERCENT OF SAID METAL BEING IN THE RANGE OF 10 TO 40 , SAID FILM ALSO INCLUDING OXYGEN AND CESIUM. 